Perception of sound: Beatings

When we have two sounds whose frequencies differ little from each other, we perceive an extra sound. This sound is similar to the other two but we can also perceive a beat whose pace is dictated by the two sounds' frequencies. If these two frequencies are too far apart, our brain no longer perceives this beating sound. This is because in order to perceive the two as a beating they both have to stimulate the hair-cells that reside in the same critical band. The beatings' frequency is equal to the number of times the two sinusoids are in phase and out of phase per second. Let's see a practical example.